The present invention relates to a process for the fractional condensation of a hot gas mixture which contains at least two condensable components and a high proportion of noncondensable components.
Hot gas mixtures which, in addition to condensable components, contain a high proportion of noncondensable components are formed, for example, in the preparation of acrylic acid or methacrylic acid by gas-phase oxidation under heterogeneous catalysis. Here, for example, propene is reacted with molecular oxygen over solid catalysts at from 200 to 400xc2x0 C. in one stage or two stages via acrolein (cf. for example DE-A-1 962 431, DE-A-2 943 707, DE-C-1 205 502, EP-A-0 257 565, EP-A-0 253 409, DE-A-2 251 364, EP-A-0 117 146, GB-B-1 450 986 and EP-A-0 293 224). Multicomponent oxidic catalysts, for example based on oxides of the elements molybdenum, bismuth and iron (in stage I) or molybdenum and vanadium (in stage II) are used. The hot reaction gas mixture formed contains, in addition to the xe2x80x9ccondensablexe2x80x9d acrylic acid or the xe2x80x9ccondensablexe2x80x9d methacrylic acid and condensable byproducts, a high proportion of noncondensable components, such as nitrogen or oxygen.
Numerous processes are known for isolating the acrylic acid. Thus, DE-C-2 136 396 discloses isolating the acrylic acid from the reaction gases obtained in the catalytic gas-phase oxidation by countercurrent absorption using a mixture of about 75% by weight of diphenyl ether and about 25% by weight of biphenyl. DE-A-2 449 780 describes the cooling of the hot reaction gas by partial evaporation of the solvent in a direct condenser (quench apparatus) before the countercurrent absorption. In addition to this absorption of the acrylic acid-containing reaction product into a high-boiling solvent mixture, other processes envisage a total condensation of acrylic acid and of the water of reaction also formed in the catalytic oxidation. The result is an aqueous acrylic acid solution which can be further worked up by distillation with an azeotropic agent (cf. for example DE-C-3 429 391 and JP-A-1 124 766) or by an extraction method (cf. for example DE-A-2 164 767 and JP-A-5 81 40-039). In EP-A-0 551 111, the acrylic acid-containing mixture prepared by means of gas-phase oxidation is brought into contact with water in an absorption tower, and the aqueous solution obtained is distilled in the presence of a solvent which forms an azeotropic mixture with polar low boilers, such as water or acetic acid. Furthermore, DE-C-2 323 328 discloses the isolation of acrylic acid from an aqueous butanol/acrylic acid esterification waste liquor by extraction with a special mixture of organic solvents. The disadvantage of the processes described here is that the extraction or absorption requires an organic solvent which has to be separated off again in a further process stage, such as rectification, with high thermal stress.
It is an object of the present invention to provide a process for separating a gas mixture formed in the catalytic gas-phase oxidation to give acrylic acid or methacrylic acid and having a high proportion of noncondensable components, so that acrylic acid or methacrylic acid is obtained in high purity and very few processes stages are required.
We have found that this object is achieved by a process for the fractional condensation of a gas mixture which, in addition to acrylic acid or methacrylic acid, also contains at least one further condensable component and additionally a high proportion of one or more noncondensable components. In the novel process for the fractional condensation, the gas mixture is passed through a column having separatory internals and the condensable components are condensed by cooling.
According to the invention, a process for the preparation of acrylic acid or methacrylic acid is also provided. Said process is based on the catalytic gas-phase oxidation of C3-/C4-alkanes, -alkenes, -alkanols and/or -alkanals and/or intermediates thereof with formation of a crude product which, in addition to acrylic acid or methacrylic acid, also contains at least one byproduct and/or at least one unconverted starting material. In the novel preparation process, the gaseous crude product is worked up by the above novel process for fractional condensation.
In a preferred embodiment, the hot gas mixture contains a high boiler fraction, medium boiler fraction and low boiler fraction, each of which in turn contains one or more components, as is the case, for example, with the hot reaction product gas mixture formed in the preparation of acrylic acid by catalytic gas-phase oxidation. Further preferred embodiments of the invention are evident from the following description and from the Example. The invention furthermore relates to the use of a column having separatory internals for the fractional condensation of hot gas mixtures containing a high proportion of noncondensable components.